Conventional continuous-wave fiber sources are typically designed to achieve high average powers, such as several 100s of W to several kW, in output beams exhibiting superior beam quality. In most cases, an output beam at or near the diffraction limit is highly desirable since it can be focused to a smaller spot resulting in higher irradiance capabilities. Due to various practical benefits, such beams are typically obtained with single-mode fiber laser system architectures using cascaded fiber gain stages. Even for cascaded fiber laser systems, however, single-mode performance is only achieved by making various tradeoffs with respect to cost, reliability, and performance.
Cascaded systems typically include a single-mode seed source, such as a single-mode fiber oscillator, one or more fiber oscillator or fiber amplifier stages coupled to the seed beam in order to scale its power, and a delivery fiber for delivering the beam to a target. In typical systems, the seed beam propagates in a core of a double clad fiber while pump light coupled into the cladding of the fiber provides an energy source for amplification of the seed beam. Subsequent amplification or delivery fiber stages use larger fiber cores to prevent the onset of detrimental non-linear effects associated with the beam and to increase pump absorption, but increasing fiber core size detrimentally allows the beam to propagate in various transverse modes higher than the preferred single, fundamental mode. Hence, various techniques are employed to maintain single-mode beam performance in large mode area (LMA) fibers.
One way to maintain single-mode performance between gain stages is to coil one or more stages, including the current, previous, or subsequent stage, so that the higher order modes are suppressed. Gain stages can also be carefully coupled to each other, such as with adiabatic fiber tapers, mode field adapters, or precision alignment so that the fundamental mode of the beam in a preceding gain stage is carefully matched or launched into the fundamental mode of the subsequent gain stage. A design in which the core diameter is simply increased with no other change in fiber parameters can cause excess splice loss because of modal mismatch between the fibers. Also, an increased core diameter can support additional modes, encouraging multimode propagation, unless the numerical aperture is decreased to compensate. In general, power coupling efficiency degradation or other optical loss associated with various gain stage couplings are necessary hindrances of achieving single-mode performance despite the increased cost of components (MFAs, thermal management) and attendant decrease in reliability from the additional components and lossy connections. A need therefore exists for innovation in high power continuous-wave systems without the aforementioned drawbacks.